1. Field of the Invention
The present invention relates to a method of producing minute metal balls having a high sphricity or roundness and a uniform diameter.
2. Description of the Related Art
Recently, minute metal balls having a high syphricity or roundness and a uniform diameter are required in a variety of fields. For example, in a field of producing electronic equipments, bonding methods called Ball Grid Array (BGA) and Chip Size Package (CSP) are carried out widely, and in these bonding methods, minute metal balls are employed. Any of the BGA and CSP is a technique in which an IC package including inputting and outputting pads arranged on a back thereof and metal balls previously mounted therein is placed on a packaging board, and the package and the packaging board are bonded by an entirely reflow.
In accordance with an increase of performance in the electronic equipment, a high-accuracy bonding size is required for bonding parts constituted the electronic equipment by the BGA, CSP or the like. Conventionally, the required bonding size indicates dimensions in longitudinal, transverse and planar directions, but in electronic parts used in certain electronic equipments, a high-accuracy bonding dimension in a height-wise direction, i.e., regarding a distance between a part and a board to which the part is bonded is also required in addition to the dimension in a planar direction. In other words, in such an electronic equipment, a junction is required not only to merely function as an electric conductive area, but also to function as a spacer.
In the connection by BGA or CSP, minute metal balls made of a Sn-based soldering alloy having a low melting point are usually employed, but in the above-described application in which the function as the spacer is required, minute metal balls made of Cu or the like having a melting point higher than a soldering alloy and coated with a soldering alloy are employed.
In the minute metal balls employed in these applications illustrated, a smaller diameter, a higher sphricity or roundness and a smaller dispersion of diameters are required altogether. Currently, such minute metal balls are produced by an oil cooling process comprising the steps of throwing small metal pieces cut at a regular size, from above, into an oil having a temperature at its upper portion higher than a melting point of the small metal pieces and a temperature at its lower portion lower than melting point of the small metal pieces, and spheroidizing the small metal pieces by utilizing a surface tension, or a process comprising the steps of heating small metal pieces to a temperature equal to or higher than its melting point, while dropping the small metal pieces freely, and spheroidizing the small metal pieces (for example, see JP-A-04-066601 and JP-A-08-295905).